The invention relates to an insert for a shaft, in particular a ventilatable sewer manhole. The said insert consists of a dish-shaped body made of impermeable material, preferably plastic, and its interior space is connected by at least one connecting duct with the part of the manhole located beneath the insert, and this connecting duct can be sealed by a shut-off device.
Underground pipe systems such as sewers, water mains, cable ducts and similar must as a rule be ventilated. Such ventilation is particularly important in the case of sewer installations because hydrogen sulphide vapours are formed here by aggressive or putrefying effluent, and if inadequate ventilation is provided these vapours will combine with water of condensation to form sulphuric acid which attacks the structures. In addition, dangerous gases frequently form in sewers and if the ventilation system is inadequate, these gases pose a serious danger to the maintenance personnel.
As a rule, the ventilation is assured by providing manholes through which access may be gained to the sewers. These manholes are sealed off at ground level by manhole covers made from concrete, steel or cast iron in which an adequate plurality of ventilation openings is provided. When rain falls, however, the disadvantage of such a design is that surface water flows through these ventilation openings into the sewer. In the event of heavy rainfall, the amounts of rainwater accumulating may be considerable. In cable ducts, the ingress of large volumes of water creates the risk of short circuits occurring. The cross sections of sewer installations are often inadequately dimensioned to carry away the sudden build-up of large amounts of surface water. Given the nowadays frequently adopted practice of employing separate sewers to handle on the one hand dirty effluent and on the other hand clean water, in particular storm water, it is absolutely essential to prevent storm water from penetrating into the sewer system carrying the dirty effluent, because on the one hand the cross sectional dimension of the latter system is not large enough to cope with such large, suddenly accumulating volumes of water, and on the other hand the sewage treatment plants provided to handle the dirty effluent, and also any pumping stations that are present, become heavily overloaded when large amounts of water suddenly pour in.
It has therefore already been proposed (in U.S. Pat. No. 4,650,365) that an insert containing a dish-shaped section made of impermeable material should be positioned below the manhole cover. The internal space of this dish-shaped section is connected with the part of the manhole below the insert, and a shut-off device is fitted in the connecting duct. At its upper edge the insert possesses a laterally projecting flange which rests on the supporting flange of a manhole frame provided for a manhole cover, and between the two flanges a seal is installed.
In this known embodiment the duct shut-off device takes the form of a pressure-release valve which is normally closed, so that no surface water can enter via the connecting duct, and the valve opens only when an overpressure builds up either in the portion of the manhole below the insert or in the sewer to which the shaft is connected. Thus, the gases generated in the sewer can escape only if an appropriately elevated pressure is reached at which the valve is caused to open. Totally unrestricted ventilation of the manhole and of the sewer connected to it, via the ventilation openings in the manhole cover, is thus not permitted by this known embodiment. Aggressive and/or dangerous gases which form are therefore not continuously vented so that the disadvantages described further above still exist with this known type of insert.
U.S. Pat. No. 4,957,389 describes a manhole insert consisting of a dish-shaped structure resting on and forming a liquid-tight seal with a support projecting from the wall of the manhole, and a drain opening, closable by a plug, is provided in the bottom of the dish. If the water collected in the dish section of the insert needs to be drained, i.e. if the dish is to be emptied, the plug has to be removed manually and once draining is completed the plug has to be replaced manually in the drain opening. In this known form of insert, the section of manhole below the insert is not ventilated to permit the escape of bad air and the inflow of fresh air.
It is the purpose of the present invention to improve a manhole insert of the type described at the beginning, namely in such a way that when large quantities of surface water occur as a result of heavy rainfall, this rainwater is substantially prevented from entering the part of the manhole below the insert, but also when the heavy rainfall ceases, continuous unrestricted ventilation of the manhole is guaranteed. In order to solve this task, the invention proposes that the ventilation duct shut-off device be activated as a function of the liquid level in the dish portion of the insert, such that when a predetermined liquid level is reached the duct is sealed. In the embodiment according to the invention, the shut-off device is generally open so that a connection exists between the space within the dish and the section of the manhole beneath the dish, and the manhole is continuously ventilated via this connection. If, in the event of heavy rainfall, a large amount of surface water penetrates into the manhole, for example via the ventilation openings in the manhole cover, this surface water collects in the dish-structure of the insert so that the liquid level rises, thereby activating the shut-off device so that no water can penetrate into the manhole via the connecting duct.
The shut-off device may be actuated in a variety of ways as a function of the liquid level in the dish; for example, sensors may be provided in the dish to detect the level of the liquid and, by means of an electrical control circuit, cause the shut-off device to operate. Preferably, however, the shut-off device takes the form of a float valve which seals off the duct when the level of liquid in the dish rises. The shut-off device may also be mechanically coupled with a separate float. However, a particularly simple design is obtained when the shut-off device consists of a preferably spherical float acting in conjunction with a valve seat arranged in the connecting duct.
In the manner according to the invention, the section of connecting duct below the shut-off device can extend upwards starting from a mouth provided near the floor of the dish. This results in a particularly simple design. If the liquid level in the dish rises and water enters into the connecting duct via its mouth, the float which forms the shut-off device is raised until it comes to rest against a valve seat, thereby sealing off the connecting duct. The section of the connecting duct above the shut-off device may be connected with an opening provided in the side wall of the dish section of the insert, and a space, via which the manhole is ventilated when the shut-off device is in the open position, is left free between the side wall of the insert and the wall of the manhole and is connected with the part of the manhole below the insert. The design may also, however, be so arranged that the section of connecting duct above the shut-off device is connected via an elbow pipe to an opening provided in the bottom of the dish. In this case, the ventilation of the section of manhole beneath the dish takes place directly via the opening in the bottom of the dish, and this opening can be of suitably large dimensions.
In one embodiment of the insert according to the invention, plurality of connecting ducts is arranged along the side wall of the dish structure. The cross sections of these connecting ducts can then be kept small, and if a shut-off device fails to operate, or operates faultily, the ingress of water is still substantially prevented because only a quantity of water proportional to the cross sectional dimension of the respective connecting duct can enter into the section of the manhole below the dish structure.
The dish should ideally be designed in such a way that even if only small quantities of surface water get in, the liquid rapidly attains a level at which the shut-off device moves into its closed position. For this purpose, it is advantageous to provide in the centre of the dish a displacement body forming an integral part of the bottom of the dish. This displacement body not only reduces the volume of the dish but also reinforces the dish, especially the bottom of the dish, and prevents the bottom from bulging or even rupturing under the weight of deposits such as sand, etc. which can penetrate into the dish via the ventilation openings.
It is also possible to provide just one single connecting duct with a mouth arranged preferably in a well in the bottom of the dish and at a distance from the side wall of the dish. This makes the dish simpler and more compact in design, which is an advantage especially in the case of small-dimensioned manholes.
Once the rainfall has stopped, the remaining water which has collected in the dish must be removed so that the water level in the dish returns to a point where the shut-off devices in the connecting ducts open up and ventilation via the connecting ducts is restored. For this purpose, in addition to the connecting duct or ducts, the interior space of the dish is connected via at least one drain opening, arranged in a known manner in the bottom of the dish, with the section of the manhole below the dish; the total cross sectional area of the drain opening(s) is smaller than that of the connecting duct or ducts. If there is any water in the dish section, a certain amount of this water will penetrate into the section of the manhole beneath the dish via this (these) drain opening(s), but because of the Small cross sectional area of the opening(s) the amount of water is small and does not cause any disadvantages. The primary purpose is, in effect, not to eliminate totally the ingress of small quantities of water, but to prevent the sudden ingress of large volumes of water per unit of time which might cause damage in the sewer or main connected to the manhole, or which might exceed the handling capacity of a sewer or main of small cross sectional dimensions, or which might temporarily impose a heavy load on sewage treatment plants and pumping stations.
It must be possible to remove the insert, which is advantageously arranged directly beneath the manhole cover, either to permit cleaning or to allow personnel to descend inside the manhole. Furthermore, water must be prevented from entering between the insert and the wall of the manhole. This goal is achieved, in a known manner, by providing a seal between a laterally projecting mounting flange at the upper end of the dish and a supporting flange projecting from the wall of the manhole. A particularly good sealing effect is obtained when the seal is formed by inflatable, flexible tubing.